1. Field of the Invention
The present invention relates to surface mounted resistors and more particularly to high power dissipating surface mounted resistors.
2. Background Information
The use of power resistors in electronic circuits is well known. Such products are produced by dozens of vendors, with ratings of a few watts up to thousands of watts, involving hundreds of physical configurations. Over the past 15-20 years there has been significant growth in the use of surface mount components, with power resistors not immune to that trend. Driving this trend has been the desire for manufacturers and consumers to have functionality in ever-decreasing equipment size or, conversely, more functionality in the same size.
The pressure to lower cost has been an additional impetus since surface mount manufacturing techniques, being highly automated in nature, are particularly conducive to high throughput and high repeatability. Electronic assembly has steadily progressed to where more and more components, previously used only in their through-hole (components mounted with their leads extending through the printed circuit board) version, are becoming available as surface mountable versions.
This is another way of saying that surface-mount assembly, originally associated with low-power circuits, is increasingly being expected to accommodate higher-power functions. Viewed from a circuit designer's standpoint, surface mount only requires a major space consideration on one side of a printed circuit board while a through-hole part has an impact on available space on both sides of the PC board: on one side for the component itself and on the other for the protrusion of leads. That is to say. a surface-mount component, aside from its many other size and reliability advantages, minimizes the need for what is called “mixed technology: that is, the use of one manufacturing process for one side of the PC board and a different process for components affixed to the other side of the PC board.
The continued acceleration of such trends in compactness and automation has led to the introduction of surface-mount power resistors by many firms. In the simplest case, the device is a standard resistor chip, with perhaps a larger size, so that it can handle up to a watt. Above that power level, many firms have adapted a mainstay of more traditional through-hole power resistor technology; that is, a winding of resistive wire, to create wire-wound surface mount types.
U.S. Pat. No. 4,672,358 owned by Ohmite discloses a power resistor with its core of traditional design but with its leads arranged for surface mounting. Similar products are offered by many firms, e.g. Vishay, IRC, KOA, Panasonic. These power resistor s are offered with dissipation up to 5 watts.
Typically, even though wire wound power resistors are wound “non-inductively,” there is some unwanted residual inductance. In high frequency applications, such inductance is a limitation. The present invention inherently minimizes the inductance.
In the U.S. Pat. No. 4,672,358 patent, the power resistor is placed down flush with the PC board surface. This is a limitation of the patent and limits the power resistor to about 50% of its stated rating. If the power resistor transmits too much heat to a circuit board, that circuit board will not pass safety-agency rating criteria. For example, a power resistor rated for a maximum temperature of 250 degrees C. (Celsius) might be operating safely at 200 degrees, but the PC board below it, being at perhaps 175 degrees C., is far above its safety-agency rating, which is typically about 100 degrees C. and 150 degrees C. for some types of fire resistant boards. U.S. Pat. No. 5,291,175 describes these considerations.
Another thermal consideration involves certain types of power resistors used in “dynamic-braking” applications. In such applications, the resistor can be subject to high current for a short period of time. U.S. Pat. No. 5,710,494 describes such applications and the principles involved. In a typical continuous-mode operation, the maximum temperature rating determines the power dissipated. If the temperature rises above that maximum, the resistor may fail. Such a rating can be influenced by the resistor surface area, the ambient temperature and cooling effects of air turbulence in the immediate vicinity of the resistor.
In a short-duration, high-current mode, however, the power dissipated in a resistor may be much higher that the listed rating and the maximum temperature not reached. This is because other parameters, namely, the mass and specific heat of the resistor control the temperature rise for short duration events. The mass acts like a shock absorber to short bursts of power. Therefore, in short-duration, high-current applications, such as dynamic braking, capacitive-discharge circuits, power supply inrush limiters etc., it is possible to have a small resistor do the job of a much larger one if the relationship of surface area, ambient temperature, air turbulence, pulse-current duration and material mass are understood wherein the temperature rise remains below the resistors rating.
In cases where it is possible to have the mass, as just referenced, in the form of a thermally conductive metal, such as aluminum or copper, it is possible to obtain substantially higher dissipation in a relatively small resistor assembly. Such higher dissipation is achieved by making use of the very low value of what is called transient thermal impedance, a term long associated with power semi-conductors. International Rectifier Corp. Application Note 949, entitled “Current Ratings of a Power Semiconductor,” 1999; and a paper by Chan Su Yun, entitled, “Static and Dynamic thermal Behavior of IGBT Power Modules,” dated Jun. 9, 2000 and available on the Internet at: www.iis.ee.ethz.ch/csyun/papers/thesis/node71.html are incorporated herein by reference. These references describe the principles involved.
A paper by the present inventor entitled, “Cooling a High Density DC-DC Converter Impacts Performance and Reliability,” published in PCIM Magazine (now Power electronics magazine), November 1999, pages 60-66 describes in detail heat removal from a power semiconductor chip whereby that heat travels from the chip through a series of barriers on its way to an ultimate cooling medium. This reference is also incorporated herein by reference. The carefully configured heat removal system described in this paper is used in preferred embodiments of the present invention to maximize power handling in both continuous and high-power transient modes.